The fastest bit rate transmittable through electromechanical cables from the deepest oil wells (10,000 meters) is a few tens of kilohertz. In contrast, the ever more sophisticated multiple-sensor probes under development have created a need for higher transmission rates. The well-known broad band characteristics of optical fibers, together with the long lengths transmissible without repeaters, make this possible. The fiber, of course, must be incorporated into an armored cable without adding significant light loss due to perturbations of the fiber ("microbends").
The problem of implementing an optical fiber transmission system arises from the very hostile environment encountered in deep drill holes. They are filled with corrosive brine, often with dissolved hydrogen sulfide. the pressure in drilling mud may be as high as 30,000 PSI. The temperature may be as high as 250.degree. C. Other limitations are that electrical power and space are at a premium in the downhole probe. It must be convenient to connect and disconnect the cable and probe. Finally, the probes are often lost downhole. Thus the transmitter cannot be inordinately expensive.
No component of the conventional optical transmission systems can function satisfactorily in the downhold environment without cooling. Semiconductor lasers and LEDs (light emitting diodes) do not operate above 100.degree. C. High pressure connectors which provide a make/break optical pathway from cable-head to probe do not exist. All plastics lose their integrity in the extreme downhole environment. Even fluorinated compounds, which are chemically inert, tend to flow under stress. An additional problem is that water penetrates plastics and promotes stress corrosion of the glass fiber.
Related prior art is disclosed in U.S. Pat. No. 4,156,104, Mondello issued May 22, 1979. While the cable described in this patent is "waterproof", it could not resist abrasion or be flexed repeatedly without fatigue failure.